John,

A SQL update trigger exposes the system table INSERTED, which contains one row for each row affected by the original UPDATE statement.

I wrote an article a while back that contains some code for this...

http://www.code-magazine.com/article.aspx?quickid=0503071&page=4

Listing 9 on that page shows how to use the contents of the INSERTED table, to query against (and update) another table.

Let me know if that helps...
KG